"If they [government troops] continue this kind of tragic attack on their own people in Aleppo, I think it ultimately will be a nail in Assad's own coffin," US Defence Secretary Leon Panetta told reporters before embarking on a week-long trip to the Middle East and Africa.

"What Assad has been doing to his own people, and what he continues to do to his own people, makes clear that his regime is coming to an end. It's lost all legitimacy. It's no longer a question of whether he's coming to an end, it's when," Panetta added.

During his visit, Panetta will hold talks with officials from Jordan, Tunisia, Egypt and Israel. He says he will use the talks to insist that Assad steps down as president, and his departure precipitates a peaceful transition to democracy in Syria.

There is still no let up in clashes in Aleppo, with Syrian forces continuing to pound rebel positions using helicopter gunships and artillery. The Syrian regime has pledged on state television that the rebels will be driven away and Aleppo will return to safety.

"Life in Aleppo has become unbearable. I'm in my car and I'm leaving right now. There's shelling night and day, every day. Bread, gasoline and gas are being sold on the black market at very high prices. Many things are in shortage," a resident in Aleppo told the Associated Press on condition of anonymity.

The UN Undersecretary-General for Humanitarian Affairs, Valerie Amos, said more than 200,000 people have already fled Aleppo, Syria's most populous city, to save their lives.

"Many people have sought temporary shelter in schools and other public buildings in safer areas. They urgently need food, mattresses and blankets, hygiene supplies and drinking water," Amos said.

Amos urged both the government and rebel forces to refrain from targeting civilians, and allow humanitarian organisations safe access.

Free Syrian Army member aims his weapon after hearing shooting in AleppoReutersA burnt bus which belonged to forces loyal to Syria President Bashar Al Assad is seen at Aleppo's district of al SakhourReutersA Free Syrian Army member shows the body of a civilian in AleppoReutersRebel fighter Mahmoud Al-Ghandour, whom opposition say was killed while fighting with forces loyal to Syria's President Bashar al-Assad, poses in front of a burning tank near AleppoReutersFree Syrian Army member walks past body of a member of Syrian President Assad's shabiha militia at Aleppo's disctrict of al-SukkariReutersFree Syrian Army member rests at Aleppo's disctrict of al SukkariReutersFree Syrian Army member runs after hearing shelling at Aleppo's district of al SukkariReutersA building burns after shelling at Aleppo's district of al SukkariReutersFree Syrian Army member shouts next to a damaged figurine at Aleppo's disctrict of al SukkariReutersFree Syrian Army member aims his weapon after hearing shelling at Aleppo's district of al SukkariReutersFree Syrian Army member carries unexploded shell to a field in AleppoReutersA sniper from the Free Syrian Army takes position in AleppoReutersA man waves as he flees AleppoReutersHelicopter belonging to Syrian President Assad's forces flies over AleppoReutersFree Syrian Army member gestures after hearing shelling at Aleppo's disctrict of Al SukariReutersFree Syrian Army members place frozen water bottles on bodies of civilians killed by shelling in AleppoReuters